1. Field of the Invention
The present invention relates to protective coatings for metal substrates. It relates more particularly, but not exclusively, to oxidation protection for titanium and titanium alloys operating at high temperatures.
2. Background of the Prior Art
Lightweight aerospace structures, including, for example, aircraft engines, gas turbines and associated compressor components, are subjected to repeated thermal cycling, between heated operational temperatures and cooler ambient temperatures. Important metals of construction for such structures may be steels, nickel based superalloys, titanium, and titanium alloys, herein referred to as metal substrates. However, excellent balance of strength, ductility, microstructural stability, and oxidation/corrosion resistance inures from low density titanium and titanium alloys, as compared with the competitive materials.
It is well known that such low density materials can improve both the efficiency of gas turbine compressors and the thrust-to-weight ratio of gas turbine engines, such as those used in aircraft. Nevertheless, the affinity of titanium for oxygen poses a serious limitation on the use of titanium and most titanium alloys, in high temperature applications, above about 600.degree. C. That is, oxygen embrittles titanium, causing a reduction in toughness and creep resistance. In addition, there are oxide coatings or scales which form on titanium, but which readily spall at above 600.degree. C.
Efforts to improve the oxidation resistance of metal substrates, especially titanium, have included the addition of alloying components such as chromium (Cr) and aluminum (Al) because such components allow formation of scales which are more protective than the titanium oxide scales. Unfortunately, the use of alloy addition alone provides inadequate oxidation protection and can adversely affect mechanical properties.
An alternative protective system or adjunct to the use of alloy additions is the use of separate and distinct layers of protective coatings applied over the titanium substrate. For example, aluminide coatings are a particularly desirable alternative because of their ability to oxidize into a highly protective Al.sub.2 O.sub.3 layer.
However, diffusion of aluminum to the underlying metal substrate seriously limits the protective ability of aluminide coatings because the loss of aluminum allows other oxides to form. Furthermore, this diffusion of Al into the substrate is accompanied by the formation of new intermetallic compounds which are deleterious to the mechanical properties of the system. Consequently, a great deal of effort has been focused on providing a means to prevent and/or compensate for the loss of Al. Ni-Cr-Al alloy coatings have been widely employed for protecting nickel and cobalt-based superalloys. One of the chief reasons that Cr is substituted for Al is that it lowers the amount of Al required to form a stable Al.sub.2 O.sub.3 layer--thus enabling the use of thinner coatings and lowering the amount of Al diffusion into the substrate. A process which, instead of compensating for Al diffusion, serves to prevent the diffusion altogether, would enable the use of even thinner coatings.
Ni-Cr-Al type coatings have been deposited on Ni superalloys, as disclosed by Galmiche, et al. in U.S. Pat. No. 4,055,706 (Oct. 25, 1977). "Duplex" processes were conducted wherein a first precasing of nickel-boron (5% to 7% boron) was deposited on the superalloy, the boron was then removed, and a second casing was deposited by chromoaluminization. Boron was employed, in the first step, to enable the use of a low-cost aqueous coating technique. However, the boron was immediately thereafter eliminated by way of an extra processing step. The high activity of boron (inter alia its affinity for oxygen), unless eliminated, had extremely adverse effects on corrosion resistance. There was some residual boron carbide and boron carbonitride remaining during this chromoaluminization second step, which was said to form a diffusion barrier. However, whatever barrier which formed had little, if any, effect in preventing outward diffusion of cobalt and inward diffusion of Al. In example la of the Galmiche patent, a cobalt-based superalloy, coated using this process, resulted in cobalt at the surface (which had thus diffused out through any barrier) and a gradient in aluminum concentration (which had thus diffused in through any barrier).
A more recent development involving Ni-Cr-Al coatings deposited on Ni-based superalloys was disclosed by Olson, et al., in U.S. Pat. No. 4,933,239 (Jun. 12, 1990). Therein, Ni and certain oxygen active materials such as yttrium, silicon and hafnium undercoated the Al-Cr.
In this case, the duplex coating process led to a thick diffusion zone which was said to improve the overall resistance to thermal-mechanical fatigue cracking while retaining an outer layer having the good oxidation resistance of M-Cr-Al-Y type coatings known from prior art (M is Ni or Co). Again, this process did not prevent Al diffusion.
The addition of Y (or Hf) to Ni-Cr-Al type coatings has been found to increase the adherence of the Al.sub.2 O.sub.3 scale and thus improve oxidation resistance. Scientific evidence suggests that impurities, such as sulfur, which diffuse from the underlying substrate, are scavenged by these reactive-element additions and that these impurities therefore do not reach the scale/aluminide interface where they would otherwise adversely affect scale adherence. In other words, the reactive impurities compensate for the outward diffusion of elements from the substrate. A process which could prevent diffusion of impurities could negate the need for Y.
The drawbacks associated with the high temperature behavior of Ni-Cr-Al or Ni-Cr-Al-Y layers coated on Ni-based superalloys are even more severe when such layers are coated on Ti substrates. Diffusion of Al into Ti is quite rapid, requiring thick coatings and leading to the formation of brittle TiAl intermetallic phases. Furthermore, TiNi intermetallic phases may also form. These intermetallic phases render the coating susceptible to spallation damage.
A. Tobin, in U.S. Pat. No. 5,049,418 (Sep. 17, 1991), coated NiAl on Ti substrates. In order to prevent brittle, degradative intermetallic TiNi and TiAl at the substrate interface, an extraneous layer of Nb and Ta was employed as a diffusion barrier. Furthermore, a special intermediate layer of copper was required in order to bond the diffusion barrier layer to the substrate.
Brindley, et al., in U.S. Pat. No. 5,116,690 (May 20, 1992) claimed that Ni-Cr-Al-Y type oxidation resistant coatings that had worked with superalloys also worked with Ti alloys. However, they did not discuss the limited high temperature lifetime which results from Al diffusion.
J. Restall disclosed in U.S. Pat. No. 5,126,213 (Jun. 30, 1992) abandonment of the duplex Ni-Cr-Al coating processes, when coating Ti substrates. Instead, he applied a singular nickel-chromium based alloy coating where nickel and chromium comprised at least 75% of the coating, allowing for only minor alloy amounts of aluminum, i.e., 2% to 12%. "Minor" proportions of boron were mentioned but not tested, and no benefit in using boron was given.
A duplex coating process which enabled a substantially greater surface content of Al and where a diffusion barrier formed in situ would represent an important advancement over the prior art.